Proteases are enzymes that cleave proteins at specific peptide bonds. Proteases can be classified into four generic classes: serine, thiol or cysteinyl, acid or aspartyl, and metalloproteases (Cuypers et al., J. Biol. Chem. 1982, 257, 7086. Proteases are essential to a variety of biological activities, such as digestion, formation and dissolution of blood clots, reproduction, and immune reaction to foreign cells and organisms. However, aberrant proteolysis is associated with a number of diseases in humans and other mammals. Accordingly, it is often beneficial to disrupt the function of one or more proteolytic enzymes in the course of treating a patient.
The binding site for a peptide substrate consists of a series of “specificity subsites” across the surface of the enzyme. The term “specificity subsite” refers to a pocket or other site on the enzyme capable of interacting with a portion of a substrate for the enzyme. In discussing the interactions of peptides with proteases, e.g., serine and cysteine proteinases, the present application utilizes the nomenclature of Schechter and Berger (Biochem. Biophys. Res. Commun. 1967, 27, 157-162). The individual amino acid residues of a substrate or inhibitor are designated P1, P2, etc. and the corresponding subsites of the enzyme are designated S1, S2, etc., starting with the carboxy terminal residue produced in the cleavage reaction. The scissile bond of the substrate is the amide bond between P1-P1′ of the substrate. Thus, for a peptide Xaa1-Xaa2-Xaa3-Xaa4, which is cleaved between the Xaa3 and Xaa4 residues, the Xaa3 residue is referred to as the P1 residue and binds to the S1 subsite of the enzyme, Xaa2 is referred to as the P2 residue and binds to the S2 subsite, and so forth.
Dipeptidyl peptidase IV (DPIV or DPPIV) is a serine protease that cleaves N-terminal dipeptides from a peptide chain containing, preferably, a proline residue in the penultimate position, e.g., in the P1 position. DPIV belongs to a group of cell-membrane-associated peptidases and, like the majority of cell-surface peptidases, is a type II integral membrane protein, being anchored to the plasma membrane by its signal sequence. DPIV is found in a variety of differentiated mammalian epithelia, endothelia and hematopoetic cells and tissues, including those of lymphoid origin where it is found specifically on the surface of CD4+ T cells. DPIV has been identified as the leukocyte differentiation marker CD26.
Proteasomes are cellular complexes comprising proteases responsible for the majority of intracellular protein turnover in eukaryotic cells, including proteolytic degradation of damaged, oxidized or misfolded proteins, as well as processing or degradation of key regulatory proteins required for various cellular functions, such as cell cycle progression. For example, the 26S proteasome is a multi-catalytic protease comprising at its catalytic core the 20S proteasome, a multi-subunit complex of approximately 700 kDa molecular weight. While serving an essential physiological role, the proteasome is also responsible for the inappropriate or accelerated protein degradation that occurs as a result or cause of pathological conditions in which normal cellular processes become disregulated. One notable example is cancer, in which the unregulated proteasome-mediated degradation of cell cycle regulatory proteins, including cyclins, cyclin dependent kinase inhibitors, and tumor suppressor genes, results in accelerated and uncontrolled mitosis, thereby promoting cancer growth and spread. (Goldberg et al. Chem. & Biol. 1995, 2, 503-508; Coux et al. Ann. Rev. Biochem., 1996, 65, 801-847; Deshaies, Trends Cell Biol. 1995, 5, 428-434). Inhibition of proteasome enzymatic function holds promise in arresting or blunting disease progression in disease states such as cancer or inflammation.
Proteasome inhibitors, e.g., lactacystin and its analogs, have been shown to block the development of the preerythrocytic and erythrocytic stages of Plasmodium spp, the malaria parasites. During both its hepatic and erythrocytic stages, the parasite undergoes radical morphological changes and many rounds of replication, events that likely require proteasome activity. Lactacystin has been found to covalently modify the catalytic N-terminal threonines of the active sites of proteasomes, inhibiting the activity of all proteasomes examined, including those in mammalian cells, protozoa, and archeae. (Gantt et al. Antimicrob. Agents Chemother. 1998, 42, 2731-2738).
The human fibroblast activation protein (FAPα) is a Mr 95,000 cell surface molecule originally identified with monoclonal antibody (mAb) F19 (Rettig et al. Proc. Natl. Acad. Sci. USA 1988, 85, 3110-3114; Rettig et al. Cancer Res. 1993, 53, 3327-3335). The FAPα cDNA codes for a type II integral membrane protein with a large extracellular domain, trans-membrane segment, and short cytoplasmic tail (Scanlan et al. Proc. Natl. Acad. Sci. USA 1994, 91, 5657-5661; WO 97/34927). FAPα shows 48% amino acid sequence identity to the T-cell activation antigen CD26, also known as dipeptidyl peptidase IV (DPP IV), a membrane-bound protein with dipeptidyl peptidase activity (Scanlan et al.). FAPα has enzymatic activity and is a member of the serine protease family, with serine 624 being critical for enzymatic function (WO 97/34927). Work using a membrane overlay assay revealed that FAPα dimers are able to cleave Ala-Pro-7-amino-4-trifluoromethyl coumarin, Gly-Pro-7-amino-4-trifluoromethyl coumarin, and Lys-Pro-7-amino-4-trifluoromethyl coumarin dipeptides (WO 97/34927).
FAPα is selectively expressed in reactive stromal fibroblasts of many histological types of human epithelial cancers, granulation tissue of healing wounds, and malignant cells of certain bone and soft tissue sarcomas. Normal adult tissues are generally devoid of detectable FAPα, but some foetal mesenchymal tissues transiently express the molecule. In contrast, most of the common types of epithelial cancers, including >90% of breast, non-small-cell lung, and colorectal carcinomas, contain FAPα-reactive stromal fibroblasts (Scanlan et al.). These FAPα+ fibroblasts accompany newly-formed tumor blood vessels, forming a distinct cellular compartment interposed between the tumor capillary endothelium and the basal aspect of malignant epithelial cell clusters (Welt et al. J. Clin. Oncol. 1994, 12, 1193-1203). While FAPα+ stromal fibroblasts are found in both primary and metastatic carcinomas, the benign and premalignant epithelial lesions tested (Welt et al.), such as fibroadenomas of the breast and colorectal adenomas, only rarely contain FAPα+ stromal cells. Based on the restricted distribution pattern of FAPα in normal tissues and its uniform expression in the supporting stroma of many malignant tumors, clinical trials with 131I-labeled mAb F19 have been initiated in patients with metastatic colon carcinomas (Welt et al.).